With time running out before funding expires for highway and transit programs, key Senate lawmakers announced a deal in principle Thursday for a new six-year highway bill — but still no final plan on how to pay for it.

“We, as leaders of this committee, have worked across party lines before the Highway Trust Fund cannot pay its bills,” Sen. Barbara Boxer, D-Calif., chairwoman of the Environment and Public Works Committee, said at a news conference.

The trust fund, the collection of gas-tax receipts that fund the nation’s surface transportation programs, is expect to run out of money later this summer.

Boxer and ranking member David Vitter, R-La., said the committee would move to mark up the bill in the next work period after the two-week spring recess.

“What we have is a detailed outline of the next highway bill in terms of policy matters within our jurisdiction,” Vitter said. “Our staff is developing the specific legislative language on all of those points. A lot of it's done, a lot of it's being worked on and we expect to be acting on that bill in the next work period after this recess.”

Sens. Thomas R. Carper, D-Del., and John Barrasso, R-Wyo., the chairman and ranking member of the committee’s Transportation and Infrastructure Subcommittee, also signed onto the proposal.

All four members said they want to fix the trust fund at some point to make it self-financing, but said there is not enough time before they must act on the next bill.

“I believe in the trust fund concept and user fees to pay for that and hopefully we can modernize that in the near future,” Vitter said.

Their comments aren’t surprising because giving up on the Highway Trust Fund would reduce the committee's influence over transportation policy.

While the Environment and Public Works Committee is primarily responsible for drafting the nation’s road construction policy, the Senate Finance Committee is responsible for coming up with how to pay for the bill. The Senate Commerce Committee has jurisdiction over transportation safety programs and the Banking Committee oversees mass transit policy.

The new six-year bill would authorize keeping current funding levels, plus inflation. Boxer said that the measure would need $16 billion a year above what is provided by the Highway Trust Fund, which is facing shortfalls in part due to improving mile-per-gallon standards, which shrinks gas tax receipts.

Boxer said she has spoken to Finance Chairman Ron Wyden, D-Ore., and said funding could be tied to a tax overhaul. Boxer and Vitter added that the likeliest offset would be repatriation — allowing corporations to bring overseas profits back into the U.S. at a lower tax rate — which would generate one-time revenue.

“There is nothing written in stone as to how this is going to be done,” Boxer said. “David mentioned repatriation. I think that is one area where there seems to be some interest in that because there are enough funds there to keep this trust fund going.”

Boxer called it a “pragmatic solution.”

Asked if there was any concern a tax overhaul may not happen in time, Boxer said the Finance Committee may be willing to do something without an overall tax rewrite, but stressed that that decision is up to Wyden and the panel.

Wyden declined to discuss any details. “I’ve been talking with [Boxer] about a variety of options, let’s just leave it at that,” he said.

Sen. Orrin G. Hatch, R-Utah, the ranking member of the Finance Committee, also said talks are ongoing.

“I don’t know that we’ve come up with any way of paying for it, but we are interesting in making sure we do what we can in that area,” Hatch said.

He was skeptical that Congress would be able to change the tax code this year, adding that renewing about 50 expired tax breaks, known as extenders, may be the best that can be done.